This invention relates to a process and an apparatus for treating photographic fixing baths, in order to remove therefrom the silver as metallic silver by electrolysis in a bath containing electrodes, i.e. at least one cathode and anodes.
Processes and apparatuses in which a fixing bath is submitted to an electrolysis so as to recover the silver in metallic form on at least one cathode are known. In these known processes and apparatuses, the electric current of electrolysis must have a sufficiently low intensity for avoiding a sulphurization of the silver. Due to the fact that the electrolysis current must be small, the recovery of the silver is far from being complete and sometimes more than 2 grams per liter of silver are lost in the drained electrolyzed fixing baths. This represents an uneconomical and of course, a substantial loss of silver.
Moreover, the statutes of the industrialized countries prohibit the drainage of photographic fixing baths containing silver salts. Although the laws vary from one country to another, it can be said that the maximum amount of silver which may be contained in the photographic fixing baths is of about 0.1 mg of silver per liter of sewage water. Practically, only baths substantially free from silver may be drained; the silver content of said baths must be measured at the outlet of the photographic development apparatus.
In a known process for treating a photographic fixing bath in order to remove therefrom the silver as a metal, the photographic fixing bath containing an excess of an anti-oxidizing agent is submitted to an electrolysis, so as to cause the deposit of metal silver onto at least one cathode placed between two anodes in the stirred electrolytic bath, the pH being maintained at about 3.5-4.5 and the electrolysis current being regulated or controlled in said bath according to its silver content, the intensity of the current being caused to decrease as the silver content of the bath diminishes. The electrolyzed bath may be used again as photographic fixing bath.
The potential difference to be applied to the electrodes during the electrolysis must comprise at least the potential difference necessary to overcome the ohmic resistance of the electrolytic cell, the concentration polarization due to the gradients of concentration in the electrolyte and the over-voltage inherent in the speed of the reactions occuring at the electrodes.
When a significant potential difference is applied to the electrodes during the electrolysis, secondary reactions occur on the electrodes, said secondary reactions being harmful to the purity and the quality of the silver deposited onto the cathode. an undesirable precipitate of silver sulfide may possibly be formed in the photographic fixing bath. The electrolysis rate is affected by the electrolytic current and consequently by the potential difference applied to the electrodes.
In order to perform the electrolysis rather quickly, avoiding secondary reactions in the photographic fixing bath at low silver concentrations, the electrolysis current is modified during said electrolysis.
Apparatus for treating photographic fixing baths are known, wherein the electric current of the electrolysis is regulated during operation, either depending on the silver concentration of the photographic fixing bath or on the resistivity of the electrolytic cell, or even depending on the overvoltage on the cathode.
These known apparatuses have the following disadvantages.
The known apparatuses measuring the concentration of silver ion are expensive and complicated.
The known apparatuses, based on the measure of the resistivity variation of the bath during the electrolysis only give a relative measure of the exhaustion of the bath. The resistivity of the electrolytic cell varies with the composition of the bath. It is therefore necessary to adjust the apparatus before each treatment as a function of the exact amount of anti-oxidant and other products added to the photographic fixing bath. Moreover, it is manadatory to keep the bath at a constant temperature, so that it is necessary to use a thermostat.
The known apparatus, based on the measure of the overvoltage at the cathode by means of a reference electrode connected by a silver nitrate bridge located about the cathode, allow the electrolysis to be controlled up to very low silver contents. However, this system implies the need of using a number of liquid junctions of various electrolytes, such as silver nitrate and for example mercury chloride, when a calomel reference electrode is used. The migration of different ions causes, during the electrolysis, irreversible phenomena which modify the over-voltage value at the electrolyte junctions.